For steel tubes and pipes produced through a plurality of production processes, the treatment in each process must be performed reliably under the predetermined production conditions, and therefore management must be carried out so that the actual goods of steel tubes and pipes can be identified. In a process such as a heat treatment process in which a plurality of steel tubes and pipes are treated at the same time, the identification management of steel tubes and pipes must be carried out in lot units, and in a process such as a grinding process and a nondestructive inspection process in which steel pipes are treated on a one pipe basis, the identification management of steel tubes and pipes must be carried out on a pipe basis. Also, at the same time, it is desired to record the production information and quality information according to each steel tube or pipe in each process and to manage the production history.
In particular, for a steel pipe for nuclear power plant use, the production history data are required to be retained for a long time period. For example, a cold finished steel pipe is produced through a plurality of production processes such as rolling, heat treatment, grinding, and inspection. And in the production processes of the steel pipe after the heat treatment process, various production histories such as heat treatment temperature, amount of grinding and result of nondestructive inspection must be managed.
Conventionally, as a method for strictly managing these production histories, there have been carried out a method in which an annexed table is attached to a pallet for moving materials, a method in which a bar code manufactured by using paper is inserted in a steel pipe, a method in which a bar code is stuck to a material-protective sheet, and the like method. In these management methods, however, much labor has been spent because production management is carried out on a pipe basis in all the processes.
For a printed wiring board used in an electric circuit, the management has been carried out manually, for example, by using a name tag, label, or process control sheet on which an item number consisting of letters of the alphanumeric characters is printed. In Patent Document 1, however, there has been proposed a method in which, in the manufacturing process of printed wiring board, a code such as a bar code or matrix code is formed from a conductor by using a printing means, etching means, or the like means, and in a subsequent process, the code is read by a commercially-available code reader.
In Patent Document 2, it is pointed out that, in the control of thread cutting process of an oil-well steel pipe produced through complicated processes of heating, rolling, finishing, heat treatment, inspection, and the like, a method in which marking is performed on the outer surface of steel pipe has a possibility of disappearing of marking, and a method in which imprinting is performed on the inner or outer surface of steel pipe has a problem of flaw generation on the steel pipe, incapability of imprinting on the inner surface of a small-diameter steel pipe, or the like. As a method for solving these problems, there has been proposed a management method in which an IC tag having the production information stored therein is buried in the end face of a protector mounted in the pipe end portion to protect the pipe body or the thread part.
In Patent Document 3, as a method for identification management of steel pipe, there has been proposed a method in which an IC tag is mounted on a lightweight holder, and the holder is stuck to the inner surface of steel pipe via a bonding medium, because in the conventional method in which a permanent magnet is used to attract an IC tag stuck to a resin plate to the inner surface of pipe, the IC tag may come off by a shock.
Also, some methods in which markings are performed directly on steel tubes and pipes by using laser processing have been proposed so far.
Patent Document 4 proposes a marking method in which a laser beam is applied to the outer peripheral surface of a metallic pipe by using a scanning mirror to draw letters, figures, and the like having a depth of 0.1 μm or smaller because much labor is required in the conventional lot control method in which an identification tag on which the lot number is written down is tied up with a nylon rope.
Patent Document 5 proposes a laser marking method of a bar code, in which thin lines are formed on the surface by scanning a laser beam, and a plurality of the thin lines are arranged to form a thick line, whereby a thermal attack and chemical change in material caused by the formation of thick line are reduced. In this method, a laser beam is applied while a steel pipe is rotated, and thereby a depth of about 10 to 50 μm is formed around the entire circumference of pipe.
Patent Document 6 proposes a fuel cladding tube having an identification code, in which the identification code such as a bar code, letters, and codes consisting of a smooth surface is formed on the fuel cladding tube that has fine unevenness on the outer surface thereof and is used for a fuel rod, and irregular reflection is made incident on the material surface and regular reflection is made incident in the identification code section by carrying out control so that the lighting angle and the imaging angle are different from each other, whereby the contrast of bright and dark between the material surface and the identification code section is increased.
Patent Document 7 discloses that for a nuclear fuel rod formed from a Zircaloy alloy, an identification code consisting of a laser beam machined bar code is read with an optical means. The invention described in Patent Document 7 relates to a nondestructive measurement method of a heat-affected zone produced by the laser beam machining, and this Document describes that even if the depth of laser beam machining is 0.05 mm, a heat-affected depth of about 0.5 mm remains.
Patent Document 8 discloses a technique in which in laser marking of a nuclear fuel cladding tube, the laser power is made low, the unevenness of a mark section or a space section other than the mark section is melted by laser heat, and the difference in reflectance between the mark section and the space section is increased, whereby the identification performance is improved because if an uneven portion is present on the surface of fuel cladding tube on account of oxidation, a change in metal crystal grains, and the like caused by machining heat, there arise a problem of a decrease in cladding capability of cladding tube and the like problems.